Arrays

Accessing array elements

You can get elements out of arrays if you know their index. Array elements' indices start at 0 and increment by 1, so the first element's index is 0, the second element's index is 1, the third element's index is 2, etc.

Syntax

array[index];

Example

var primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37]; primes[0]; // 2 primes[3]; // 7 primes[150]; // undefined

Array literals

You can create arrays in two different ways. The most common of which is to list values in a pair of square brackets. JavaScript arrays can contain any types of values and they can be of mixed types.

Syntax

const arrayName = [element0, element1, ..., elementN]

Example

const primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37];

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Multi-dimensional Arrays

A two-dimensional array is an array within an array. If you fill this array with another array you get a three-dimensional array and so on.

Example

// Two dimensions, 3x3 const multidimensionalArray = [[1,2,3],[4,5,6],[7,8,9]]

Array constructor

You can also create an array using the Array constructor.

Example

const stuff = new Array(); stuff[0] = 34; stuff[4] = 20; stuff; // [34, undefined, undefined, undefined, 20]

Example

const myArray = new Array(45 , 'Hello World!' , true , 3.2 , undefined); console.log(myArray); // output: [ 45, 'Hello World!', true, 3.2, undefined ]

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Accessing nested array elements

Accessing multi-dimensional array elements is quite similar to one-dimension arrays. They are accessed by using [index][index]..... (number of them depends upon the number of arrays deep you want to go inside).

Syntax

array[index][index] //...

Example

const myMultiArray = [ [1, 2, 3, 4, 5, [1, 2, 3, 4, 5] ], [6, 7, 8, 9, 10, [1, 2, 3, 4, 6] ], [11, 12, 13, 14, 15, [1, 2, 3, 4, 5] ], [16, 17, 18, 19, 20, [1, 2, 3, 4, 5] ] ]; console.log(myMultiArray[1][5][4]); // Outputs 6, the value in the last element of the last element of the second element of myMultiArray.

Booleans

Boolean literals

Syntax

true false

Boolean logical operators

Syntax

expression1 && expression2 // Returns true if both the expressions evaluate to true expression3 || expression4 // Returns true if either one of the expression evaluates to true !expression5 // Returns the opposite boolean value of the expression

Example

if (true && false) {; // This block is not entered because the second expression is false } if (false || true) { // This block is entered because any one of the expression is true } if (!false) { // This block is entered because !false evaluates to true } !!true // Evaluates to true

Example

if (!false && ( false || (false && true) )) { console.log('Guess what...'); } /* Not executed because !false && ( false || (false && true) ) - becomes !false && ( false || false) - becomes true && false , which is false. */

Example

/* An important thing to note here is the Operator Precedence - which determines the order in which operators are evaluated. Operators with higher precedence are evaluated first. Thus among the four - () , && , || , ! */ // Brackets - have the highest precedence // ! - lower than Brackets // && - lower than ! // || - the lowest if (true && !!false || true) { console.log('Guess again ??'); } /* Executed , here is the evaluation process- true && !!false || true - becomes true && false || true - (no brackets present , so ! evaluated ) becomes false || true - (then && evaluated) which becomes true */

Example

/* Next important thing is the Associativity - which determines the order in which operators of the same precedence are processed. For example, consider an expression: a * b * c. Left-associativity (left-to-right) means that it is processed as (a * b) * c, while right-associativity (right-to-left) means it is interpreted as a * (b * c). */ // Brackets , && , || have left to right associativity // ! has right to left associativity !false && !!false // false // evaluated in the manner - !false && false - true && false - false

Comparison operators

Syntax

x === y // Returns true if two things are equal x !== y // Returns true if two things are not equal x <= y // Returns true if x is less than or equal to y x >= y // Returns true if x is greater than or equal to y x < y // Returns true if x is less than y x > y // Returns true if x is greater than y

"Truthy" and "Falsy"

Only Boolean literals (true and false) assert truth or false, but there are some other ways too to derive true or false. Have a look at the examples.

Example

if (1) { // Output 'True!', since any non-zero number is considered to be true console.log('True!'); } if (0) { // Not executed, since 0 evaluates to falsy console.log('I doubt if this gets executed'); } if ('Hello') { // Gets executed because non-empty strings are truthy console.log('So, any non-empty String is also true.'); } if ('') { // Not executed console.log('Hence , an empty String is false'); }

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== vs. ===

A simple explanation would be that == does just value checking (no type checking ), whereas, === does both value checking and type checking. Seeing the examples may make it all clear. It is always advisable that you never use ==, because == often produces unwanted results.

Syntax

expression == expression expression === expression

Example

'1' == 1 // true (same value) '1' === 1 // false (not the same type) true == 1 // true (because 1 evaluates as truthy, though it's not the same type) true === 1 // false (not the same type)

Code Comments

Code comments are used for increasing the readability of the code.If you write 100 lines of code and then forget what each function did, it's not useful at all. Comments are like notes, suggestions, warnings, etc. that you can put for yourself. Code comments are not executed

Single Line Comment

Anything on the line following // will be a comment while anything before will still be code.

Syntax

console.log('This code will be run'); // console.log('Because this line is in a comment, this code will not be run.') // This is a single line comment.

Multi-Line Comment

Anything between /* and */ will be a comment.

Syntax

/* This is a multi-line comment! */

Example

/* console.log('Hello,I won't be executed.'); console.log('Hello ,I also will not be executed'); */

Console

console.log

Prints text to the console. Useful for debugging.

Example

const name = 'Codecademy'; console.log(name);

console.time

This function starts a timer which is useful for tracking how long an operation takes to happen.You give each timer a unique name, and may have up to 10,000 timers running on a given page.When you call console.timeEnd() with the same name, the browser will output the time, in milliseconds, that elapsed since the timer was started.

Syntax

console.time(timerName);

Example

console.time('My Math'); const x = 5 + 5; console.log(x); console.timeEnd('My Math'); console.log('Done the math.'); /* Output: 10 My Math: (time taken) Done the math. */

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console.timeEnd()

Stops a timer that was previously started by calling console.time().

Syntax

console.timeEnd(timerName);

Example

console.time('My Math'); const x = 5 + 5; console.log(x); console.timeEnd('My Math'); /* Output : 10 My Math: (time taken) */

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Functions

A function is a JavaScript procedure—a set of statements that performs a task or calculates a value.It is like a reusable piece of code. Imagine , having 20 for loops ,and then having a single function to handle it all . To use a function, you must define it somewhere in the scope from which you wish to call it. A function definition (also called a function declaration) consists of the function keyword, followed by the name of the function, a list of arguments to the function, enclosed in parentheses and separated by commas, the JavaScript statements that define the function, enclosed in curly braces, { }.

Syntax

function name(argument1 , argument2, /* ..., argumentN */){ statement1; statement2; // ... statementN; }

Example

function greet(name) { return 'Hello' + name + '!'; }

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Function calling

Syntax

functionName(argument1, argument2, ..., argumentN);

Example

greet('Anonymous'); // Hello Anonymous!

Function hoisting

The two ways of declaring functions produce different results. Declaring a function one way "hoists" it to the top of the call, and makes it available before it's actually defined.

Example

hoistedFunction(); // Hello! I am defined immediately! notHoistedFunction(); // ReferenceError: notHoistedFunction is not defined function hoistedFunction () { console.log('Hello! I am defined immediately!'); } var notHoistedFunction = function () { console.log('I am not defined immediately.'); }

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If statement

It simply states that if this condition is true, do this, else do something else (or nothing). It occurs in varied forms.

if

Syntax

// Form : Single If if (condition) { // code that runs if the condition is true }

Example

if (answer === 42) { console.log('Told you so!'); }

else

A fallback to an if statement. This will only get executed if the previous statement did not.

Syntax

// If the condition is true, statement1 will be executed. // Otherwise, statement2 will be executed. if (condition) { // statement1: code that runs if condition is true } else { // statement2: code that runs if condition is false }

Example

if (animal == 'dog') { console.log('Bark, bark!'); } else { console.log('Meow!'); }

else if

This is like an else statement, but with its own condition. It will only run if its condition is true, and the previous statement's condition was false.

Syntax

// Form : else if. If the condition is true, statement1 will be executed. Otherwise, condition2 is checked. if it is true, then statement2 is executed. Else, if nothing is true, statement3 is executed. if (condition1) { statement1; } else if (condition2) { statement2; } else { statement3; }

Example

if (someNumber > 10) { console.log('Numbers larger than 10 are not allowed.'); } else if (someNumber < 0) { console.log('Negative numbers are not allowed.'); } else { console.log('Nice number!'); }

Loops

For Loops

You use for loops, if you know how often you'll loop. The most often used varName in loops is i.

Syntax

for ([let i = startValue]; [i < endValue]; [i+=stepValue]) { // Loop code here }

Example

for (let i = 0; i < 5; i++) { console.log(i); // Prints the numbers from 0 to 4 }

Example

let i; // 'outsourcing' the definition for (i = 10; i >= 1; i--) { console.log(i); // Prints the numbers from 10 to 1 }

Example

/* Note that all of the three statements are optional, i.e. , */ let i = 9; // This loop is perfectly valid: for(;;){ if(i === 0) { break; } console.log(i); i--; }

While Loops

You use while loops, if you don't know how often you'll loop.

Syntax

while (condition) { // Your code here }

Example

let x = 0; while (x < 5) { console.log(x); // Prints numbers from 0 to 4 x++; }

Example

let x = 10; while (x <= 5) { console.log(x); // Won't be executed x++; }

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Do While Loops

You use do while loops, if you have to loop at least once, but if you don't know how often.

Syntax

do { // Your code here } while (condition);

Example

let x = 0; do { console.log(x); // Prints numbers from 0 to 4 x++; } while (x < 5);

Example

let x = 10; do { console.log(x); // Prints 10 x++; } while (x <= 5);

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Math

.random()

Returns a random number between 0 and 1.

Syntax

Math.random()

Example

Math.random(); // A random number between 0 and 1.

.floor()

Returns the largest integer less than or equal to a number.

Syntax

Math.floor(expression)

Example

Math.floor(9.99); // 9 Math.floor(1 + 0.5); // 1 Math.floor(Math.random() * x + 1); // Returns a random number between 1 and x

.pow()

Returns base raised to exponent.

Syntax

Math.pow(base, exponent)

Example

Math.pow(2,4); // gives 16

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.ceil()

Returns the smallest integer greater than or equal to a number.

Syntax

Math.ceil(expression)

Example

Math.ceil(45.4); // 46 Math.ceil(4 - 1.9); // 3

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.PI

Returns the ratio of the circumference of a circle to its diameter, approximately 3.14159 or in better terms, the value of PI (π). Note in syntax , we do not put () at the end of Math.PI because Math.PI is not a function.

Syntax

Math.PI

Example

Math.round(Math.PI); // rounds the value of PI ,gives 3 Math.ceil(Math.PI); // 4

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.sqrt()

Returns the square root of a number.

Syntax

Math.sqrt(expression)

Example

Math.sqrt(5+4); // 3 Math.sqrt(Math.sqrt(122+22) + Math.sqrt(16)); // 4

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Numbers

% (Modulus)

It returns the remainder left after dividing the left hand side with the right hand side.

Syntax

number1 % number2

Example

14 % 9 // returns 5

isNaN()

Returns true if the given number is not a number, else returns false.

Syntax

isNaN([value])

Example

const userInput = prompt('Enter a number'); // "Enter a number" if (isNaN(userInput)) { console.log('I told you to enter a number.'); } // console.log executed since 'a number' is not a number // Another important thing: if (isNaN('3')) { console.log('bad'); } // Not executed because the string "3" gets converted into 3, and 3 is a number

Basic Arithmetic

Doing basic arithmetic is simple.

Syntax

4 + 5; // 9 4 * 5; // 20 5 - 4; // 1 20 / 5; // 4

Prefix and Postfix increment/decrement operators

Prefix increment / decrement operators are operators that first increase the value of the variable by 1 (increment) or decrease the value of an expression / variable by 1 (decrement) and then return this incremented / decremented value. They are used like ++(variable) [increment] or --(varaible) [decrement] On the other hand , Postfix increment / decrement operators are operators that first return the value of the variable and then increase the value of that variable by 1 (increment) or decrease the value of the variable by 1 (decrement) . They are used like (variable)++ [increment] or (varaible)-- [decrement].

Syntax

// Prefix Decrement: --variable // Prefix Increment: ++variable // Postfix Decrement: variable-- // Postfix Increment: variable++

Example

// The examples will make it clear let x = 15; // x has a value of 15 const y = x++; // since it is postfix , the value of x (15) is first assigned to y and then the value of x is incremented by 1 console.log(y); // 15 console.log(x); // 16 let a = 15; // a has a value of 15 const b = ++a; // since it is prefix , the value of a (15) is first incremented by 1 and then the value of x is assigned to b console.log(b); // 16 console.log(a); // 16

Objects

Object Literals

Syntax

{ 'property 1': value1, property2: value2, number: value3 }

Example

var obj = { name: 'Bob', married: true, 'mother\'s name': 'Alice', 'year of birth': 1987, getAge: function () { return 2012 - obj['year of birth']; }, 1: 'one' };

Property Access

Syntax

name1[string] name2.identifier

Example

obj['name']; // 'Bob' obj.name; // 'Bob' obj.getAge(); // 24

OOP

Classes

A class can be thought of as a template to create many objects with similar qualities. Classes are a fundamental component of object-oriented programming (OOP).

Syntax

SubClass.prototype = new SuperClass();

Example

const Lieutenant = function (age) { this.rank = 'Lieutenant'; this.age = age; }; Lieutenant.prototype = new PoliceOfficer(); Lieutenant.prototype.getRank = function () { return this.rank; }; const John = new Lieutenant(67); John.getJob(); // 'Police Officer' John.getRank(); // 'Lieutenant' John.retire(); // true

Popup boxes

alert

Display an alert dialog with the specified message and an OK button. The alert dialog should be used for messages which do not require any response on the part of the user, other than the acknowledgment of the message.

Syntax

alert(message);

Example

alert('Hello World');

confirm

Displays a dialog with the specified message and two buttons, OK and Cancel.

Syntax

confirm('message') // Returns true if confirmed, false otherwise

Example

if (confirm('Are you sure you want to delete this post?')) { deletePost(); }

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prompt

The prompt() displays a dialog with an optional message prompting the user to input some text. If the user clicks the "Cancel" button, null is returned.

Syntax

prompt(message);

Example

var name = prompt('Enter your name:'); console.log('Hello ' + name + '!');

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Strings

Strings are text. They are denoted by surrounding text with either single or double quotes.

Syntax

"string of text" 'string of text'

Concatenation

Syntax

string1 + string2

Example

'some' + 'text'; // returns 'sometext' const first = 'my'; const second = 'string'; const union = first + second; // union variable is the string 'mystring'

.length

Returns the length of the string.

Syntax

string.length

Example

'My name'.length // 7 , white space is also counted ''.length // 0

.toUpperCase(), .toLowerCase()

Changes the cases of all the alphabetical letters in the string.

Example

'my name'.toUpperCase(); // Returns 'MY NAME' 'MY NAME'.toLowerCase(); // Returns 'my name'

.trim()

Removes whitespace from both ends of the string.

Syntax

string.trim()

Example

' a '.trim(); // 'a' ' a a '.trim(); // 'a a'

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.replace()

Returns a string with the first match substring replaced with a new substring.

Example

'original string'.replace('original', 'replaced'); // returns 'replaced string'

.charAt()

Returns the specified character from a string. Characters in a string are indexed from left to right. The index of the first character is 0, and the index of the last character in a string called stringName is stringName.length - 1. If the index you supply is out of range, JavaScript returns an empty string.

Syntax

string.charAt(index) // index is an integer between 0 and 1 less than the length of the string.

Example

'Hello World!'.charAt(0); // 'H' 'Hello World!'.charAt(234); // ''

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.substring()

Returns the sequence of characters between two indices within a string.

Syntax

string.substring(indexA[, indexB]) // indexA : An integer between 0 and the length of the string // indexB : (optional) An integer between 0 and the length of the string.

Example

'adventures'.substring(2,9); // Returns 'venture' // substring starts from indexA(2) , and goes up to but not including indexB(9) 'hello'.substring(1); // returns 'ello' 'Web Fundamentals'.substring(111); // returns '' 'In the market'.substring(2,999); // returns ' the market' 'Fast and efficient'.substring(3,3); // returns '' 'Go away'.substring('abcd' , 5); // returns 'Go aw' // Any non-numeric thing is treated as 0

.indexOf()

Returns the index within the calling String object of the first occurrence of the specified value, starting the search at fromIndex, Returns -1 if the value is not found. The .indexOf() method is case sensitive.

Syntax

string.indexOf(searchValue[, fromIndex]) // fromIndex is optional. It specifies from which index should the search start.Its default value is 0.

Example

'My name is very long.'.indexOf('name'); // returns 3 'My name is very long.'.indexOf('Name'); // returns -1 , it's case sensitive 'Where are you going?'.indexOf('are', 11); //returns -1 'Learn to Code'.indexOf(''); //returns 0 'Learn to Code'.indexOf('', 3); //returns 3 'Learn to Code'.indexOf('', 229); returns 13 , which is the string.length

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Switch statements

Acts like a big if / else if / else chain. Checks a value against a list of cases, and executes the first case that is true. It goes on executing all other cases it finds after the first true case till it finds a breaking statement, after which it breaks out of the switch If it does not find any matching case, it executes the default case.

Syntax

switch (expression) { case label1: statements1; // break; case label2 statements2; // break; ... case labelN: statementsN; // break; default: defaultStatement; // break; }

Example

const weather = "clear"; switch (weather) { case 'clear': console.log("Beautiful day!"); case 'cloudy': console.log('I wish the sun were out!'); default: console.log('Some weather, huh?'); } // Because there are no breaks, all three statements will be logged

Ternary Operator

The ternary operator is usually used as a shortcut for the if statement.

Syntax

condition ? expr1 : expr2

Example

const grade = 85; console.log('You ' + (grade > 50 ? 'passed!' : 'failed!')); //Output: You passed! /* The above statement is same as saying: if(grade > 50){ console.log('You ' + 'passed!'); // or simply 'You passed!' } else{ console.log('You ' + 'failed!'); } */

Variables

Variable Assignment

Syntax

var name = value;

Example

var x = 1; var myName = 'Bob'; var hisName = myName;

ES6 const and let

Syntax

// let variables can be reassigned let message = 'I can be reassigned!'; // const variables cannot be reassigned const message2 = 'I cannot be reassigned';

Scope

let and const are both scoped to the block in which they are initialized. This can be a smaller scope than var variables, which are scoped to the function in which they are initialized.

const dog = true; if (dog) { let mood = 'happy' } console.log(mood) // Error! mood is scoped to the if block, so it is not defined outside that block.

Variable Reassignment

Syntax

varname = newValue

Example

// Declare variable and give it value of 'Michael' let name = 'Michael' // Change the value of name to 'Samuel' name = 'Samuel'

const variables cannot be reassigned and attempting to do so will throw a TypeError:

Example

// Declare variable and give it value of 'Michael' const name = 'Michael' // Try changing the value of name to 'Samuel' name = 'Samuel' // Throws: // TypeError: Assignment to constant variable.
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